Dyes are intensely colored substances used for the coloration of various substrates, including paper, leather, fur, hair, foods, drugs, cosmetics, plastics, and textile materials. They are retained in these substrates by physical adsorption, salt or metal-complex formation, solution, mechanical retention, or by the formation of covalent bonds. The methods used for the application of dyes to the substrates differ widely, depending upon the substrate and class of dye. It is by application methods, rather than by chemical constitutions, that dyes are differentiated from pigments. During the application process, dyes lose their crystal structures by dissolution or vaporization. The crystal structures may in some cases be regained during a later stage of the dyeing process. Pigments, on the other hand, retain their crystal or particulate form throughout the entire application procedure. They are usually applied in vehicles, such as paint or lacquer films, although in some cases the substrate itself may act as the vehicle, as in the mass coloration of polymeric materials.
The principal usage or application classes of dyes accounting for 85% of production in the United States are as follows: acid dyes, basic dyes, direct dyes, disperse dyes, fluorescent brighteners, reactive dyes, sulfur dyes, and vat dyes.
Dyeing describes the imprintation of a new and often permanent color, especially by impregnating with a dye, and is generally used in connection with textiles, paper, and leather. Printing may be considered as a special dyeing process by which the dye is applied in locally defined areas in the form of a thickened solution and then fixed.
Generally, dyes are dissolved or dispersed in a liquid medium before being applied to a substrate where they are fixed by chemical or physical means, or both. Owing to its suitability, its availability, and its economy, water usually is the medium used in dye application; however, nonaqueous solvents have been studied extensively in recent years.
Textile substrates can be classified in three groups: cellulosic, protein, and synthetic polymer fibers. Economical and uniform distribution of a small amount of dye throughout the substrate and fixation of the dye are the keys to dyeing, i.e., with regard to fastness to washing and to other deteriorating influences. It is the fixation of the dye to a substrate to which the present invention is directed.
The production of dyeings of acceptable quality requires the use of many auxiliary products and chemicals. These include chemicals that improve fastness properties such as bleaching agents, wetting and penetrating agents, leveling and retarding agents, and lubricating agents. Other agents are used to speed the dyeing process or for dispersion, oxidation, reduction, or removal of dyes from poorly dyed textiles.
Dyes of similar or identical chromophoric class are used for widely differing applications and, therefore, are classified according to their usage rather than their chemical constitution. Dyes with identical or similar solubilizing groups generally display similar dyeing behavior even though their main structure may vary substantially. Another important consideration in the use of a given dye for a specific application and fastness properties of commercial dyes is found in the pattern cards issued by their manufacturers. The following classification of colorants for dyeing is used: acid, basic, direct, disperse, insoluble azo, sulfur, vat, fiber-reactive, miscellaneous dyes, and pigments.
The most common types of fibers to be dyed with acid dyes are polyamide, wool, silk, modified acrylic, and polypropylene fibers, segmented polyester-polyurethane, as well as blends of the aforementioned fibers with other fibers such as cotton, rayon, polyester, regular acrylic, etc. Approximately 80-85% of all acid dyes sold to the U.S. textile industry are used for dyeing nylon, 10-15% for wool, and the balance for those fibers mentioned above. Acid dyes are organic sulfonic acids; the commercially available forms are usually their sodium salts, which exhibit good water solubility.
The two major polyamide types commercially available today are nylon 6, and nylon 6,6. Both fiber types are typically very receptive to acid dyes under certain conditions. A direct relationship exists between the chemical structure of an acid dye and its dyeing and wetfastness properties. The dyeing process is influenced by a number of parameters, such as: dyestuff selection, type and quantity of auxiliaries, pH, temperature and time.
Affinity and diffusion are fundamental aspects of the dyeing process. The former describes the force by which the dye is attracted by the fiber, and the latter describes the speed with which it travels within the fiber from areas of higher concentration to areas of lower concentration.
In the application of dyes, there have developed over the years three chief principles of dyeing textiles. In one case, the dye liquor is moved as the material is held stationary. In another case, the textile material is moved without mechanical movement of the liquor. Examples of the foregoing include jig dyeing and continuous dyeing which involves the padding of the fabric. A combination of the two is exemplified by a Klauder-Weldon skein-dye machine in which the dye liquor is pumped as the skeins are mechanically turned. Another example is a jet or spray dyeing machine in which both the goods and the liquor are constantly moving.
A substantially non-mechanical dyeing process is typically referred to as exhaustion. This process involves the preparation of a dye bath containing an aqueous solution, usually water, and the dye. The textile to be dyed is then inserted into the dye bath. The temperature of the dye bath is then raised to a predetermined optimal level, with the pH of the bath being similarly maintained, and the textile material is then soaked in the bath. During this soaking process, the dye contained in the bath is absorbed into the fibers of the textile material in accordance with the principles of affinity and diffusion as described above. Once all of the dye has been absorbed, the bath is referred to as being exhausted, with only the aqueous solution being left.
The selection of proper dyeing equipment depends on the nature and volume of the material to be dyed. Raw stock and yarns are dyed by exhaust methods, whereas fabrics are dyed both by exhaust or continuous methods. The choice of method for fabrics depends largely on the volume to be dyed. Continuous dyeing is usually employed where the volume of fabric for a particular shade is about 10,000 yards or more.
In the dyeing of fabrics, the beck is one of the oldest dyeing machines known. It consists of a tub containing the dye liquor, and an elliptical winch or reel which is located horizontally above the dye bath. Ten or more pieces of fabric are dyed simultaneously. Each piece is drawn over the winch, and its two ends are sewn together to form an endless rope. The ropes are kept in the dyeing machine side by side, separated from each other by rods to prevent them from tangling. During the dyeing process the reel rotates, pulling the ropes out of the dye bath and dropping them back into the dye bath at the opposite side. In this way almost all the fabric is kept inside the dye bath.
Becks are used for dyeing knits and other light-weight fabrics that can be easily folded into a rope form without causing damage. Fabrics made of filament yarns that tend to break should not be dyed in a beck since the broken filaments will dye deeper. Very light fabrics should also be avoided as they may tend to float on the dye bath and tangle.
Jet dyeing machines are similar to becks in that the fabric is circulated through the dye bath in the rope form. However, in a jet the transportation of the fabric occurs by circulating the dye liquor through a venturi jet, instead of the mechanical pull of the reel in a beck. The fabric is pulled out of the main dyeing chamber by means of a high speed flow of dye liquor that passes through the venturi opening.
Modern jet dyeing machines are generally categorized as "round kier" or "cigar kier" configurations. Most fabrics can be dyed satisfactorily in conventional round kier dyeing machines such as the Gaston 824 jet dyeing machine. These types of machines operate at low liquor ratio and yield very good results on most fabrics. However, certain fabrics have more of a tendency to develop crush or pile marks due to their constructions.
Padders are used to impregnate fabrics with liquors containing dyes, dyeing assistants or other chemicals. Padding is usually followed continuously by other treatments, from drying to a series of successive treatments. The simplest padder consists of two parts: the trough containing the dye liquor, and two squeezing rollers arranged above the dye liquor. In the padding process, the fabric in its open width form, enters the trough through tension rails, passes through the dye liquor, and is then squeezed between two heavy rubber rollers with the proper hardness, under pressure. Excess dye liquor runs back into the trough.
Impregnation is typically followed by drying during which dye migration becomes a major concern. Evaporating water tends to carry with it dye particles from wet spots to dry spots on the fabric, and from the inside or back to the face of the fabric, and may lead to uneven and/or shading problems. To prevent migration, drying is done gradually, and/or a chemical migration inhibiting agent may be used to treat the dyed substrate.
Once the dyed substrate is sufficiently dried, the dye must then be fixed to the substrate so to preclude its bleeding from the substrate. One method of achieving this is through the use of a fixation oven. These ovens are used when fixation of the dyes is performed with dry heat. Both hot flue or heated cans are used for this purpose. Since temperatures as high as 215.degree. C. are often required, the cans are heated with hot oil or gas. Contact heating, as with heated cans, has the advantage that less time is required for the fixation process as compared to the use of dry air.
Another method of fixing dyes to a substrate is by treating the substrate with a dye fixative which similarly improves the wetfastness of a dyed textile by precluding the dye from bleeding or migrating out of the textile material after it comes in contact with water. For example, it is desirable that an article of dyed clothing retain its color while it is being washed using various laundry detergents, whether in a washing machine or by hand. Similarly, when rain water and the like comes in contact with a dyed article of clothing, the retention of the dye within the fibers of the material, rather than its migration onto other substrates is highly desirable. It is to these types of aftertreatments for these particular purposes to which the present invention is directed.
The reason that a dye fixative may be necessary is dependent on the type of acid dye being employed. For example, those acid dyes that offer excellent dyeing characteristics such as good leveling, migration, and coverage of barre, have only marginal wetfastness properties. Conversely, those acid dyes that provide high wetfastness do not level very well. Obviously, the employment of the first type of acid dyes requires the use of a fixing additive to improve the relatively poor wetfastness properties of those dyes. However, it is oftentimes also desirable to further enhance the wetfastness properties of dyes already adequate in their wetfastness ability.
A number of fixing agents or dye fixatives currently being used in the industry contain formaldehyde and phenols. The environmental disadvantages associated with their use are well known. However, another serious disadvantage associated with their use in combination with dyed materials is their tendency to discolor the dyed material due to a chemical reaction between the phenols and the dye. Consequently, this results in a substantial financial loss of product and resources.
Therefore, there is a need to provide a process for fixing dyes absorbed in synthetic textile materials which is more environmentally friendly than the currently used fixatives containing phenols and formaldehyde, while at the same time significantly decreasing the occurrence of discoloration of dyed synthetic substrates upon application of the dye fixative in order to improve the wetfastness and colorfastness of the dyed finished products.
The present invention provides a process for the fixing of dyes contained in synthetic textile materials in just such a manner.